tokio/util/
rand.rs

1cfg_rt! {
2    mod rt;
3    pub(crate) use rt::RngSeedGenerator;
4
5    cfg_unstable! {
6        mod rt_unstable;
7    }
8}
9
10/// A seed for random number generation.
11///
12/// In order to make certain functions within a runtime deterministic, a seed
13/// can be specified at the time of creation.
14#[allow(unreachable_pub)]
15#[derive(Clone, Debug)]
16pub struct RngSeed {
17    s: u32,
18    r: u32,
19}
20
21/// Fast random number generate.
22///
23/// Implement `xorshift64+`: 2 32-bit `xorshift` sequences added together.
24/// Shift triplet `[17,7,16]` was calculated as indicated in Marsaglia's
25/// `Xorshift` paper: <https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf>
26/// This generator passes the SmallCrush suite, part of TestU01 framework:
27/// <http://simul.iro.umontreal.ca/testu01/tu01.html>
28#[derive(Clone, Copy, Debug)]
29pub(crate) struct FastRand {
30    one: u32,
31    two: u32,
32}
33
34impl RngSeed {
35    /// Creates a random seed using loom internally.
36    pub(crate) fn new() -> Self {
37        Self::from_u64(crate::loom::rand::seed())
38    }
39
40    fn from_u64(seed: u64) -> Self {
41        let one = (seed >> 32) as u32;
42        let mut two = seed as u32;
43
44        if two == 0 {
45            // This value cannot be zero
46            two = 1;
47        }
48
49        Self::from_pair(one, two)
50    }
51
52    fn from_pair(s: u32, r: u32) -> Self {
53        Self { s, r }
54    }
55}
56
57impl FastRand {
58    /// Initialize a new fast random number generator using the default source of entropy.
59    pub(crate) fn new() -> FastRand {
60        FastRand::from_seed(RngSeed::new())
61    }
62
63    /// Initializes a new, thread-local, fast random number generator.
64    pub(crate) fn from_seed(seed: RngSeed) -> FastRand {
65        FastRand {
66            one: seed.s,
67            two: seed.r,
68        }
69    }
70
71    #[cfg(any(
72        feature = "macros",
73        feature = "rt-multi-thread",
74        feature = "time",
75        all(feature = "sync", feature = "rt")
76    ))]
77    pub(crate) fn fastrand_n(&mut self, n: u32) -> u32 {
78        // This is similar to fastrand() % n, but faster.
79        // See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
80        let mul = (self.fastrand() as u64).wrapping_mul(n as u64);
81        (mul >> 32) as u32
82    }
83
84    fn fastrand(&mut self) -> u32 {
85        let mut s1 = self.one;
86        let s0 = self.two;
87
88        s1 ^= s1 << 17;
89        s1 = s1 ^ s0 ^ s1 >> 7 ^ s0 >> 16;
90
91        self.one = s0;
92        self.two = s1;
93
94        s0.wrapping_add(s1)
95    }
96}
tokio/util/rand.rs

tokio/util/
rand.rs
